Dissociative Adsorption and Desorption Processes of Cl 2 /GaAs(001) Surfaces
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CALCULATION METHODS We perform first-principles total-energy calculations based on density functional theory. The total energy functional is minimized with respect to both the plane-wave coefficients of the occupied orbitals and the ionic degrees of freedom by using the conjugate gradient technique [2]. Beyond the local density approximation, we include the nonlocal exchange and correlation effects within the generalized gradient approximation of Perdew and Wang [3]. The generalized gradient approximation has been found to improve the description of the adsorption and desorption processes of molecules on solid surfaces [4]. We employ ab initio norm-conserving pseudopotentials of the Kleinman-Bylander type [5]. The pseudo wavefunctions are expanded in terms of a plane-wave basis set corresponding to a kinetic-energy cutoff of 7.29 Ry. Four special k points are employed to sample the primitive surface Brillouin zone. 451 Mat. Res. Soc. Symp. Proc. Vol. 408 01996 Materials Research Society
The GaAs(001) surface exhibits various reconstructions depending on processing conditions [6,7]. We consider the Ga-rich (4x2) and the As-rich (2x4) surfaces in this paper, both of which have three dimers and one dimer vacancy within the surface unit cell. These two GaAs(001) surfaces obey the electron counting rule; all Ga dangling bonds are empty and all As dangling bonds are fully filled. The GaAs(001) surface is modeled using supercell geometry with a (4x2) or (2x4) surface unit cell containing a six layer GaAs slab. The dangling bonds on the bottom surface of the slab are terminated with hypothetical hydrogen atoms [8]. The (4x2) and (2x4) surface unit cells correspond to a C12 -C1 2 separation of 8.0 A, which implies a weak interaction between C12 molecules in adjacent supercells. The substrate atoms in the topmost four GaAs layers are fully relaxed for every fixed position of the molecule which is adsorbed or desorbed. This is because the substrate atoms have time to move appreciably during dissociation and desorption events, due to the small mass ratio between Ga, As, and Cl atoms, in contrast to the case of the H2 dissociation [4]. It is found that a supercell containing a six layer GaAs slab and an energy cutoff of 7.29 Ry are sufficient to achieve the convergence of energy differences within 0.1 eV.
RESULTS AND DISCUSSIONS First, we consider the dissociative adsorption of a C12 molecule on the Ga-rich GaAs(001)(4x2) surface with three Ga dimers on the top layer. Figure 1 presents the potential energy curves for the C12 dissociation as a function of the Cl-Cl bond length, for various values of the height of the C12 molecule above the surface. The Cl 2 molecule is approaching over the center Ga dimer with its axis kept parallel to the surface as shown in Fig. 1. It is found that the potential energy of the C12 molecule with its original bond length reduces as the C12 height decreases, which indicates that the C12 molecule is attracted to the surface. This is because the Ga dangling-bond orbitals sticking out into vacuum
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